It is known in the prior art to bond wafers using an intermediate material, such as a glass frit, or a metal-to-metal bond. Bonding with a glass frit requires a large bond footprint, which increases the area and cost of the bond. Bonding with a glass frit may also introduce contaminants into a cavity between the bonded wafer, which may degrade the performance of the wafers being bonded or any devices fabricated in or on a wafer.
Metal-to-metal bonding using aluminum and sputtered germanium has been constrained by the need for a dedicated tool or chamber, which adds cost and complexity to the bonding process. Some bonds require that the aluminum layer be considerably thicker than the germanium layer, and that the germanium layer is formed near the end of the fabrication process, which constraints add cost and complexity to the process.